Collections

List

• A List is an ordered collection of elements.

• Elements can be accessed by their index.

• Java provides several implementations of the List interface:

  • ArrayList: A resizable array that is backed by an array.

  • LinkedList: A doubly-linked list that allows for fast insertion and deletion at the beginning or end of the list.

  • Vector: Similar to ArrayList but is synchronized, which makes it thread-safe.

• Example:

List<String> names = new ArrayList<>();
names.add("John");
names.add("Jane");
names.add("Joe");
System.out.println(names.get(0));

Set

• A Set is an unordered collection of unique elements.

• Java provides several implementations of the Set interface:

  • HashSet: Uses a hash table to store the elements, which provides constant time performance for basic operations.

  • TreeSet: Stores the elements in a sorted order, which provides faster searching.

  • LinkedHashSet: Maintains the insertion order of the elements, which provides predictable iteration order.

• Example:

Set<String> names = new HashSet<>();
names.add("John");
names.add("Jane");
names.add("Joe");
System.out.println(names.contains("John"));

Map

• A Map is a collection of key-value pairs.

• Keys must be unique, but values can be duplicated.

• Java provides several implementations of the Map interface:

  • HashMap: Uses a hash table to store the elements, which provides constant time performance for basic operations.

  • TreeMap: Stores the elements in a sorted order based on the keys.

  • LinkedHashMap: Maintains the insertion order of the elements, which provides predictable iteration order.

• Example:

Map<String, Integer> scores = new HashMap<>();
scores.put("John", 10);
scores.put("Jane", 20);
scores.put("Joe", 30);
System.out.println(scores.get("Jane"));

Queue

• A Queue is a collection that represents a waiting line.

• Elements are added to the end of the queue and removed from the front.

• Java provides several implementations of the Queue interface:

  • LinkedList: A doubly-linked list that allows for fast insertion and deletion at the beginning or end of the list.

  • PriorityQueue: A queue that orders its elements according to their natural order or a specified comparator.

• Example:

Queue<String> queue = new LinkedList<>();
queue.add("John");
queue.add("Jane");
queue.add("Joe");
System.out.println(queue.peek());

Stack

• A Stack is a collection that represents a stack of items.

• Elements are added to the top of the stack and removed from the top.

• Java provides a Stack class that implements the Stack interface.

• Example:

Stack<String> stack = new Stack<>();
stack.push("John");
stack.push("Jane");
stack.push("Joe");
System.out.println(stack.pop());

Collection utilities

• Java provides several utility classes for working with collections:

  • Collections: Provides methods for sorting, searching, and manipulating collections.

  • Arrays: Provides methods for working with arrays, such as sorting and searching.

• Example:

List<String> names = Arrays.asList("John", "Jane", "Joe");
Collections.sort(names);
System.out.println(names);